Methods for making holographic reticles for characterizing optical systems

ABSTRACT

Characterization of an optical system is quickly and easily obtained in a single acquisition step by obtaining image data within a volume of image space. A reticle and image plane are positioned obliquely with respect to each other such that a reticle having a plurality of feature sets thereon, including periodic patterns or gratings, is imaged in a volume of space, including the depth of focus. Metrology tools are used to analyze the detected or recorded image in the volume of space through the depth of focus in a single step or exposure to determine the imaging characteristics of an optical system. Focus, field curvature, astigmatism, spherical, coma, and/or focal plane deviations can be determined. The present invention is particularly applicable to semiconductor manufacturing and photolithographic techniques used therein, and is able to quickly characterize an optical system in a single exposure with dramatically increased data quality and continuous coverage of the full parameter space. In embodiments, the test reticle is holographically generated by interfering two or more beams of optical radiation. The resulting interference pattern is recorded on a reticle and used for testing the optical system. The geometry of the holographic interference pattern is tightly controlled by the properties of the interfering beams and is therefore more accurate than conventional reticle writing techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.09/907,902, filed Jul. 19, 2001, which is a continuation-in-part of U.S.application Ser. No. 09/339,506, filed Jun. 24, 1999, each of which isincorporated herein in its entirety by reference. This application alsoclaims the benefit of U.S. Provisional Application No. 60/219,187, filedJul. 19, 2000, which is also incorporated herein in its entirety byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to characterizing an optical system, andparticularly to the rapid and precise characterization of an opticalsystem including focus, field curvature, astigmatism, spherical, coma,and/or focal plane deviation using holographically produced reticles.

2. Background Art

Photolithography is often used in the manufacture of semiconductordevices and other electronic equipment. In photolithography, projectionoptics of high quality are often used to image features on a reticleonto a photosensitive substrate, such as a resist covered wafer. As thefeature sizes desirable to be reproduced become ever smaller, theoptical system or projection optics must be continually maintained andchecked for image quality.

Often, the performance of an optical system or projection optics isdifficult to obtain without time consuming techniques. Generally,multiple exposures are required of a photosensitive substrate atdifferent locations in the image field and at different focus depths tocharacterize the optical system. The optical system is thencharacterized by compiling information obtained from examining themultiple processed images. Each of the many exposures and thecorresponding processed images are acquired serially. Consequently,focus errors, scan errors and temporal variations to the optical systemparameters during the measurement are compounded.

In the case of scan and focus errors, noise is introduced into the data.In the case of temporal variations, valid data are unrecoverable.Additionally, the data are discretely sampled rather than continuousacross the parameter range. Consequently, quantization errors resultfrom estimation of data values that lie between adjacent samples.

With demand for increasing production throughput and increasingperformance requirements of the projection optics capable of imagingreduced feature size, there is a need for improving the apparatus andmethods used to characterize an optical system. There is also a need todevelop an apparatus and method that will quickly and easily providehigh-precision data or information that can be used to characterize theperformance of an optical system quickly and easily and with dataobtained simultaneously and processed simultaneously without the need toperform multiple exposures and processing of multiple images.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises a method and apparatus for obtainingoptical system characterization information simultaneously by utilizinga volume of space during a relatively short time or in a singleexposure. A test reticle having a plurality of features with differentorientations, sizes, and line types is imaged with the optical systembeing characterized. Either the object plane in which the reticle ispositioned or the image plane in which the characterization data isobtained is tilted or angled within the corresponding three-dimensionalvolume of space. The reticle, having a plurality of features, is imagedwith the optical system being characterized. In a volume of space,through a depth of focus, an envelope of feature quality through focusis thereby obtained. This envelope of feature quality is simultaneouslyobtained by acquiring image data of the reticle in a plane that isoblique to the reticle plane. The resulting image of the reticle andcorresponding features are analyzed with metrology techniques, which caninclude an interferometric tool thereby obtaining optical systemcharacteristics. The optical system characteristics that can be obtainedinclude focus, field curvature, astigmatism, coma, distortion,telecentricity and/or focal plane deviation, as well as information onspherical aberrations and variation of coherence.

In embodiments, the test reticle described above is producedholographically. More specifically, a holographic reticle is generatedby interfering two or more beams of optical radiation to generate aninterference volume having periodic interference pattern(s). Theinterference patterns are recorded on a reticle blank using any of thevarious recording techniques, such as photographic films, photo-resist,etc. The geometry of the periodic interference patterns is tightlycontrolled by the properties of the interfering optical beams. Morespecifically, the geometry is controlled by the wavelength of light, thewavefront variation, and the geometry of the exposure configuration(i.e., the relative beam angle of the optical radiation before and afterinterference). All of these factors can be controlled much moreprecisely than serially written e-beam or laser writing tools.Additionally, much larger reticle areas can be written in a single passusing holographic patterning. As such, writing errors that result fromstitching together e-beam sub-fields are avoided entirely.

Accordingly, it is an advantage of the present invention that an opticalsystem is characterized quickly and in a single exposure or imagingoperation.

It is an advantage of the present invention that it results in a rapidacquisition of data required for characterization of an optical system.

It is another advantage of the present invention that it results inrapid acquisition of data desensitized to the focus, scan, and temporalerrors associated with prior techniques.

It is an advantage of the present invention that reticle writing errorsare reduced or eliminated by holographically patterning the testreticle, as compared to e-beam or laser reticle writing tools.

It is an advantage of the present invention that holographicallypatterned reticles can print linewidths that are much smaller thancurrent reticle writing tools.

It is an advantage of the present invention that pitch uniformity forperiodic gratings can be tightly controlled by holographicallypatterning test reticles. For example, chirped or continuously variablepitch patterns can be produced with great accuracy. This affords theprobing of optical system performance over a precisely-controlledcontinuum of linesizes, line orientations, and pattern pitches.

It is an advantage of the invention that phase shifts in periodicstructures can be precisely controlled. Phase shift structures arevaluable in the characterization of odd optical aberrations that producefeature shifts in the image plane.

It is a feature of the present invention that information or data isobtained throughout a volume of image space.

It is another feature of the present invention that the reticle is in adifferent plane than the plane from which data is acquired in imagespace.

It is yet another feature of the present invention that theperpendicular from the reticle and/or image plane interceptor benon-collinear with the axis of the optical system.

These and other objects, advantages, and features will be readilyapparent in view of the following detailed description.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1A is a schematic illustration of a photolithographic system.

FIG. 1B is a schematic illustration of a photolithographic system with ademodulating device 24.

FIG. 2 is a perspective view of the reticle or object space.

FIG. 3 is a perspective view of the photosensitive substrate or imagespace.

FIG. 4 is a plane view illustrating a test reticle having a plurality ofperiodic structures or patterns thereon.

FIG. 5A is a plane view illustrating one type of grating or periodicpattern or structure.

FIG. 5B is a plane view illustrating another type of grating or periodicpattern or structure.

FIG. 6 schematically illustrates the acquisition of data used tocharacterize an optical system.

FIG. 7 is a block diagram illustrating high-level method steps of anembodiment of the present invention.

FIG. 8A schematically illustrates a volume of space.

FIG. 8B is a schematic plane view of an image formed on a photosensitivesubstrate.

FIG. 9A is a schematic plane view of one embodiment of a portion of apattern on a reticle.

FIG. 9B is a schematic perspective view illustrating detection ofastigmatism based upon the embodiment illustrated in FIG. 9A.

FIG. 10A is a schematic plane view illustrating a reticle.

FIG. 10B is a schematic plane view illustration a portion of a reticlepattern.

FIG. 11A is a schematic plane view of another embodiment of a portion ofa pattern on a reticle.

FIG. 11B is a schematic perspective view illustrating detection ofastigmatism based upon the embodiment illustrated in FIG. 11A.

FIG. 12 is a schematic plane view illustrating a portion of a reticlepattern utilized in detecting spherical aberrations.

FIG. 13 is a schematic plane view illustrating a reticle divided intodifferent feature sets or pattern portions for detecting differentaberrations used in characterizing the optical system.

FIG. 14 is a perspective graphical view of an interferometer mapillustrating detection of distortion or aberrations of an optical systemin an embodiment of the present invention.

FIG. 15 is a graph illustrating the different distortions or aberrationsthat can be detected with an embodiment of the present invention.

FIGS. 16A-D graphically illustrate, in perspective, the differentdistortions or aberrations illustrated in FIG. 15.

FIG. 17 is a plane view of a photosensitive substrate illustrating anembodiment of the present invention used to obtain best focus of anoptical system.

FIG. 18 is a graph illustrating detection of spherical aberrations in anembodiment of the present invention.

FIG. 19A is a schematic plane view illustrating an embodiment of thepresent invention for determining optimum placement of a reticle forenhanced imaging.

FIG. 19B is a schematic plane view of a reticle utilized in theembodiment of the present invention illustrated in FIG. 19A.

FIG. 20 illustrates a holographic reticle writing system according to anembodiment of the present invention.

FIG. 21 illustrates a flowchart for writing a holographic reticleaccording to an embodiment of the present invention.

FIG. 22A illustrates spherical two beam interference produced byholographic patterning according to an embodiment of the presentinvention.

FIG. 22B illustrates the pitch uniformity for spherical two beaminterference according to an embodiment of the present invention.

FIG. 23A illustrates a system for generating a chirped grating usingholographic patterning according to an embodiment of the presentinvention.

FIGS. 23B-D illustrate various chirped gratings produced by holographicpatterning according to an embodiment of the present invention.

FIG. 23E illustrates a circular zone plate array.

FIG. 23F illustrates focus determination of an optical system using aninterlaced chirped grating according to an embodiment of the presentinvention.

FIG. 24 illustrates an atomic force micrograph of a cross-grating thatwas holographically patterned on a test reticle according to anembodiment of the present invention.

FIG. 25 illustrates a holographic hex pattern that was produced byholographic patterning according to an embodiment of the presentinvention.

FIG. 26 illustrates a polygonal grating produced by holographicpatterning according to an embodiment of the present invention.

FIG. 27 illustrates a zone plate array that was produced by holographicpatterning according to an embodiment of the present invention.

FIG. 28 illustrates holographically patterned gratings that collectivelydepict pitch change and phase change according to an embodiment of thepresent invention.

FIG. 29A illustrates a reticle with a holographic pattern having aconstant pitch grating with a varying duty cycle.

FIGS. 29B and 29C illustrate how the holographic pattern of reticle 2900is formed.

FIG. 30 illustrates a phase control system for accurately controllingphase shift of a holographically produced grating according to anembodiment of the present invention.

FIG. 31 illustrates a holographic reticle writing system with accuratephase shift control according to an embodiment of the present invention.

FIG. 32 illustrates a flowchart 3200 for varying the phase shift of aholographically generated grating according to an embodiment of thepresent invention.

FIG. 33 illustrates an interferogram according to an embodiment of thepresent invention.

FIG. 34 illustrates a graph of distortion verses optical linewidth foran optical system under test according to an embodiment of the presentinvention.

FIGS. 35A and 35B illustrate how partial coherence affects the imageoffsets.

FIG. 36 illustrates how the measurement of the relative image shifts asa function of linewidth can be simplified by using a reticle withvarious linewidths such that each linewidth is an integral multiple of afundamental linewidth size.

The preferred embodiments of the invention are described with referenceto the figures where like reference numbers indicate identical orfunctionally similar elements.

DETAILED DESCRIPTION OF THE INVENTION

1. Characterizing Optical Systems

FIG. 1A schematically illustrates the present invention. Aphotolithographic system 10 is generally illustrated. An illuminationsource 12 is used to project the image of a reticle 16 within a reticleor object space or volume 14 onto a photosensitive substrate 22 within aphotosensitive substrate or image space 20 through optical system orprojection optics 18. The reticle 16 is positioned within a plane thatis oblique with respect to the photosensitive substrate 22. The reticle16 and the photosensitive substrate 22 can be tilted in a variety ofdifferent ways. Preferably, the positioning of the reticle 16 or thewafer 22 is such that either the reticle 16 or wafer 22 extends throughthe object volume or depth of focus of the optical system or projectionoptics 18. The imaging data recorded by the photosensitive substrate 22provides information permitting the characterization of the opticalsystem or projection optics 18. Imaging characteristics such as focus,field curvature, astigmatism, coma, and/or focal plane deviation, aswell as information for determining spherical aberration and variationof coherence can be obtained. The image quality of the entire imagefield through focus can be obtained in a single imaging or exposureoperation in a relatively short time. The entire image of the reticlecan be analyzed with metrology techniques for characterizing the opticalsystem or projection optics 18. The optical system or projection optics18 is thereby characterized in the x and y field direction as well asthe depth of focus in the z direction. While a photosensitive substrate22 has been indicated as a way to record the electromagnetic radiationpassing through the reticle 16, any device for detecting electromagneticradiation may be used, for example a photoreceptive sensor, such as acharge coupled device (CCD) array, position sensitive detector (PSD), orequivalent detector.

Alternatively, a demodulating device can be used to produce aninterference pattern. FIG. 11B is a schematic illustration of aphotolithographic system with a demodulating device 24. The demodulatingdevice 24 could be a demodulating reticle, an electro-optic demodulatingdevice, an acousto-optic demodulating device, or another demodulatingdevice as would be known to one skilled in the art.

Advantageously, the interference pattern can be visually observed todetect optical aberrations and to support adjustments to thephotolithographic system 10 in real time.

The interference pattern could include a Moire fringe pattern. The Moirepattern could represent a global magnification change and localdistortion changes from a nominal pattern.

The demodulating device 24 could be followed by a photosensitivesubstrate 22, a CCD array, a PSD, or an equivalent detector as describedabove. Alternatively, the detector array could be patternedlithographically or holographically to generate an integrated detectormodule. The multiplexing of parameter space, as described below, can beused to detect focus, astigmatism, coma distortion, magnification, etc.,without the need for expensive and time-consuming photographic recordingsystems.

FIG. 2 illustrates an object space or reticle space 114, which is anexample of object 14 in FIG. 1A. Placed within the object or reticlespace 114 is a reticle 116 comprised of a plurality of differentperiodic features 116 a, 116 b, 116 c, 116 d, and 116 e. Each of theplurality of different periodic patterns or features 116 a, 116 b, 116c, 116 d, or 116 e can contain a grating pattern of varying line types,shapes, sizes, and orientations for obtaining different imaginginformation or data for characterizing the optical system. The periodicfeatures or structures need only be periodic and need not be gratings.The reticle 116 can be tilted within the object or reticle space 114 byan angle 124. Accordingly, the reticle 116 is positioned within thereticle or object space 114 over a range of depth z₁.

FIG. 3 is a perspective view illustrating a photosensitive substrate 122angularly positioned in a data acquisition plane of the photosensitivesubstrate image or space 120. The photosensitive substrate 122 ispositioned at an angle 126 within the photosensitive substrate or imagespace 120. The photosensitive substrate 122 extends through a range ofdepth z₂ This range of depth z₂ is within and beyond the depth of focusof the optical system or projection optics. The photosensitive substrate122 is illustrated tilted at angle 126 that is compound to the angle oftilt 124 of the reticle 116, illustrated in FIG. 2. It should beappreciated that the reticle 116 and the photosensitive substrate 122can be angled or tilted in different ways with respect to each other andthe tilts illustrated in FIGS. 2 and 3 are only illustrative of thepossible tilt or angle that can be utilized in the present invention. Inobtaining useful characterization data for an optical system accordingto the teachings of the present invention, it may only be necessary tohave one plane oblique with respect to the other plane, with the degreeand nature of the oblique positioning of the two planes determinedsolely by the type and quantity of characterizing data desired. Forexample, the plane of the reticle need not be tilted, while the plane ofthe photosensitive substrate is tilted or made oblique to that of theplane of the reticle. Alternatively, one skilled in the art wouldrecognize that the above description of the configuration relationshipbetween the reticle and the photosensitive substrate could also apply toa similar configuration relationship between the reticle and thedemodulating device 24 described above with the explanation of FIG. 11B.

FIG. 4 is a plane view illustrating a reticle 216 having a plurality ofdifferent periodic features, patterns, structures, or gratings thereon.Reticle 216 is an example of reticle 16 in FIG. 1A. The differentperiodic features can be grouped forming different feature sets, whichcan be used to obtain different imaging information for characterizingthe optical system. For example, reticle 216 can be comprised of aplurality of different line types, shapes, sizes, and orientations thatcan make up the four feature sets. For example, a first feature set 216a comprising a basket weave, a second feature set 216 b comprising aplurality of horizontal and vertical lines, a third feature set 216 ccomprising a plurality of horizontal and vertical lines having differentspacing or sizing relative to the second feature set 216 b, a fourthfeature set 216 d comprising a different set of horizontal and verticallines, and a fifth feature set 216 e comprising a basket weave, whichcan be the same or different than the first feature set 216 a. Thereticle 216 comprises a plurality of different feature sets, which caninclude different lines and spacing or gratings over the entire imagefield for imaging onto an oblique plane within the object space. Thedetection and analyzing of the image in a plane traversing the imagespace results in the acquisition of optical system characterizationdata, which can be utilized to determine the performance or imagingcharacterizations of the optical system.

FIG. 5A is an example of another feature set 316 c, which can be placedon a portion of a reticle and imaged onto a photosensitive substrate.The feature set 316 c can be comprised of a central field having widthw₁, that is comprised of multiplex or interlaced rows or stripes forminga pattern. For example, row 330 has spaced vertical lines thereon, row332 has spaced horizontal lines thereon, row 334 has spaced negative 45degrees tilted lines thereon, and row 336 has positive 45 degrees tiltedlines thereon. The stripes or rows 330, 332, 334, and 336 can form apattern extending along the length L of the feature set 316 c formed ona portion of a reticle, as illustrated in FIG. 5A. The edges of thefeature set 316 c can be formed from a column or a vertical stripe 328.Formed within columns 328 is a basket weave pattern. The basket weave incolumns 328 can be formed from partially transmissive sections orportions. The entire width of the feature set 316 c is w₂. By way ofexample, the feature set 316 c can have dimensions of approximately 27mm in length L with the total width w₂ of approximately 5 mm and thecenter width w₁ being approximately 4.5 mm. Each row or stripe can beapproximately 50 microns high or wide. Each linewidth within the row canbe in the order of 200 nanometers. The feature set illustrated in 316 cis given only by way of an example. Other feature sets can be utilizedto determine the characteristics of an optical system, without departingfrom the spirit and scope of the present invention.

FIG. 5B illustrates another feature set 316 d, which can be utilized ona portion of a reticle. Feature set 316 d comprises a pattern ofhorizontal, vertical and angled lines. Stripe or row 330′ has a verticalline pattern thereon. Row or stripe 332′ has a plurality of horizontallyspaced lines thereon. Row or stripe 334′ has a plurality of lines tiltednegative 45 degrees and row or stripe 336′ has a plurality of linestilted positive 45 degrees thereon. The plurality of rows or stripes arerepeated in a horizontal, vertical, negative 45 degrees, positive 45degrees pattern along the length of the feature set 316 d. Other rows orpatterns can be placed within a feature set depending upon thecharacteristic of the optical system desired to be detected ordetermined.

FIG. 6 illustrates the processing of information obtained from theimaging of the reticle with the optical system or projection optics tobe characterized. An image plane 420 is detected or recorded onto aphotosensitive substrate. The image plane has a plurality of imagescomposed of the feature set images 420 a, 420 b, 420 c, 420 d, and 420 eimaged by a reticle, such as that illustrated in FIG. 4. Data obtainedfrom the image plane 420, which is positioned obliquely to the reticleplane, is extracted over the entire image field plane, which ispreferably recorded onto a photosensitive substrate using a metrologicaltool 40, which is preferably an interferometer. The metrological tool 40can detect or extract information, such as interference patterns,determined or detected from the image of the feature sets on thereticle. The images are formed on the image plane 420 and can berecorded on a photosensitive substrate. Alternatively, the images formedon the image plane 420 can be viewed in real time by using ademodulating device, such as demodulating device 24 as shown in FIG. 1B.Signal processor 42, coupled to the metrological tool 40, analyzes andprocesses the different images of the different feature sets 420 a, 420b, 420 c, 420 d, and 420 e. The processed signals from the signalprocessor 42 are provided to an optical system characterizor 44.Different aberrations of the optical system can therefore be determined.For example, astigmatism can be determined as a function of best focusdifference of periodic pattern or grating orientation. Coma can bedetermined as a function of second order distortion signature versusfocus. Spherical aberration can be determined as a function of bestfocus difference between line sizes versus field position. The recordeddata can be analyzed by different metrology tools, such as white light,a dark field microscope, a large aperture interferometer, a lasermicroscope interferometer, or a interferometric microscope, for example.

FIG. 7 is a block diagram illustrating high-level method steps of anembodiment of the present invention. A step 510 represents imaging areticle having a periodic grating or pattern thereon in a plane obliqueto the reticle plane with an optical system being characterized. Theperiodic pattern can be comprised of different grating patterns, witheach different grating pattern designed to determine a predeterminedcharacteristic or property of the of the optical system. A step 512represents recording data representing the image of the periodic patternor grating detected in the plane oblique to the reticle plane. The imageof the periodic pattern or grating can be recorded with a photosensitivesubstrate or by electronic means, or presented for viewing in real timeby using a demodulating device, as would be apparent to one skilled inthe relevant art. A step 514 represents interferometrically analyzingthe recorded data to determine the imaging properties of the opticalsystem. The data representing the periodic pattern, grating or gratingsis analyzed with interferometric techniques to obtain the properties ofthe optical system. The optical system can be characterized over theentire field and at different depths of focus in a single operation.

FIGS. 8-13 illustrate the application of the concepts of the presentinvention to different embodiments for characterizing an optical systemby determining different optical properties, such as field curvature anddifferent aberrations, including astigmatism and spherical aberrations.

FIG. 8A illustrates a volume of space 620, which is an example of volumeof space 20 in FIG. 1A. Within the volume of space 620, electromagneticradiation representing images can be detected. For example, generally aphotosensitive substrate 622 or a demodulating device (such asdemodulating device 24 as shown in FIG. 1B) is positioned within thevolume of space 620 at an angle θ from the x-y plane. An image from anoptical system (such as system 10 from FIG. 1A) is projected onto thephotosensitive substrate 622. The image projected onto thephotosensitive substrate 622 is that of a plurality of feature sets orspaced lines placed on a reticle, as illustrated in the prior figures.The use of a photosensitive substrate 622 is illustrative of thepreferred embodiment; however, it should be appreciated that any photoreceptor or demodulating device can be placed in the volume of space 620to receive and detect the electromagnetic radiation representing animage of the reticle.

FIG. 8B illustrates the detection of field curvature utilizing aphotosensitive substrate 622 or demodulating device (not shown)positioned as illustrated in FIG. 8A. Line 631 represents the fieldcurvature for the optical system being characterized and the width d ofline 631 represents the depth of focus of the optical system beingcharacterized. Accordingly, by tilting a photosensitive substrate 622within a volume of space 620 and using a reticle having a plurality offeatures that are imaged onto the photosensitive substrate, the fieldcurvature and depth of field can quickly and easily be determined. Byselecting the appropriate features and orientations on a reticle,additional information characterizing the optical system can be obtainedin a single exposure of a photosensitive substrate or single dataacquisition of the receipt of electromagnetic radiation within thevolume of space.

The line 631 can be created with a periodic pattern or grating reticleimaged on a tilted photosensitive substrate 622. A periodic pattern orgrating strip or line 631 will be produced down the center of the field.The line 631 should be calculated to be narrow enough to define thecentral strip of the field, but wide enough to include severalresolvable points in the direction of the x axis. This is a function ofthe pixel density of the detector array, charge coupled device, orposition sensitive detector used to view the strip or line 631. Aphase-shifting interferometer can be used. Data can be obtained bypositioning the photosensitive substrate 622 at the Littrow angle withrespect to the phase-shifting interferometer. The Littrow angle is theangle at which electromagnetic radiation from the interferometerretro-diffracts to return to the interferometer. The peaks of anintensity map acquired by the phase-shifting interferometer are thepoints of best focus of the optical system being characterized. Thesepeaks comprise a ridge in the direction of the y axis. The meandering ofthis ridge in the direction of the x axis as the field is traversed inthe direction of the y axis represents the field curvature. Therobustness of this procedure relies on its ability simultaneously toacquire intensity data at points throughout the volume of space 620.Calibration, scaling, and extraction of data are straightforward. Thismethod uses the intensity of the retro-diffraction. Field curvature canalso be detected using the phase of the retro-reflection. In thismethod, the photosensitive substrate is positioned perpendicular to thephase-shifting interferometer axis. The acquired phase map consists ofthe feature resist height at each point on the photosensitive substrate.In the direction of the x axis, the quality of feature is a function offocus curve. In the direction of the y axis, the shift of best focus forany feature size and orientation is a function of field position. Thefield curvature and astigmatism can be extracted from the comparison ofcurve shift as a function of orthogonal feature orientation, as would beapparent to one skilled in the relevant art.

FIGS. 9A and 9B schematically represent detection of astigmatismaccording to the present invention. FIG. 9A schematically represents apattern that can be repetitively reproduced on a reticle or a mask orpresented for viewing by a demodulating device (such as demodulatingdevice 24 as shown in FIG. 1B) for use in detecting astigmatism. Portion716 contains orthogonal gratings or line patterns. Vertical lines 730are interlaced or alternate between horizontal lines 732. The verticallines 730 and the horizontal lines 732 are mutually perpendicular withrespect to each other.

FIG. 9B represents the image formed on a photosensitive substrate or acorrespondingly configured demodulating device (not shown) that has beentilted in the volume of space, such as that illustrated in FIG. 8A. Thefeature set or portion of the periodic patterns or gratings 716′ imagedon the photosensitive substrate has a lateral dimension f representingthe depth of field. Across the dimension f, representative of the depthof field, different image quality will be obtained with the best imagequality being located at the highest point along dimension f. Anenvelope 735 is formed. The envelope 735 represents the image quality ina dimension f along the depth of focus of the recorded image 732′ of thehorizontal line 732 illustrated in FIG. 9A. Similarly, the verticallines 730 illustrated in FIG. 9A are represented by recorded image 730′.An envelope 733 is formed representing the image quality of the depth offocus for the recorded image 730′, of the vertical lines 730 on theportion 716 of a reticle, illustrated in FIG. 9A. The best image qualitybeing graphically represented by the highest point along the envelopes733 and 735. Any astigmatism in the optical system at the image locationis represented by distance a, which represents the different imaging ofthe horizontal and vertical lines. The axial separation of thetangential and sagittal image planes can be detected by the differentpoints of focus represented by the envelopes 733 and 735. The lateralshift of these different points of focus is represented by distance a.

Many different feature sets or periodic patterns or gratings can beutilized according to the present invention. FIGS. 10A and 10Billustrate another feature set, periodic pattern, or grating that can beutilized in determining astigmatism of an optical system. FIG. 10A is aplane view illustrating a reticle or mask 817 having a plurality ofstripes 816, each stripe 816 containing a reticle pattern or featureset. FIG. 10B schematically illustrates one of the reticle periodicpatterns or gratings 816 from which the reticle 817 illustrated in FIG.10A is formed. The feature set, periodic pattern or grating 816 isformed from a plurality of columns of periodic patterns or gratings.Pairs of adjacent columns of periodic patterns or gratings are formedfrom pairs of orthogonal lines. For example, column 830 is formed fromvertical lines and column 832 is formed from horizontal lines. Thehorizontal and vertical lines are orthogonal. Column 836 is formed froma +45 degrees tilted line and column 834 is formed from a −45 degreestilted line. Therefore, the lines in columns 836 and 834 are orthogonal.The interlacing of columns having different line orientations, as isillustrated in FIG. 10B, provides information as to the aberrations inthe optical system being characterized. The aberrations in a substantialportion of the field can be detected simultaneously in practicing thepresent invention.

FIGS. 11A and 11B are simplified schematic representations illustratingthe use of lines or feature sets to determine astigmatism according tothe present invention. In this embodiment of the present invention,lines or feature sets are arranged in columns rather than rows. FIG. 11Aillustrates a plane view of a portion of a reticle pattern 916. Thereticle pattern is formed from a plurality of feature sets or lines, aportion of which is formed by columns of lines that alternate betweenhorizontal and vertical orientations. Columns 930 are formed from aplurality of vertical lines and columns 932 are formed from a pluralityof horizontal lines. The image formed from portion 916 of a reticle,when projected in image space, can be used to detect astigmatism. Inthis embodiment, a photosensitive substrate utilized to record the imageof the reticle portion 916 is tilted with respect to the reticle portion916 out of the x-y plane and rotated about the y axis. Alternatively, ademodulating device (such as demodulating device 24 as shown in FIG. 1B)can be correspondingly configured with respect to the reticle. FIG. 1Bschematically represents the detection and analysis of the image in theimage space to determine astigmatism at the field location. Because thephotosensitive substrate on which the image is recorded is tilted out ofthe x-y plane and rotated about the y axis, the x direction representsthe depth of focus, as illustrated in FIG. 11B. The height in the zdirection, illustrated by FIG. 11B, represents the image quality at adifferent depth of focus. Bars 930′, in FIG. 11B, represent the imagequality of the alternating columns 930 of vertical lines illustrated inFIG. 11A. The image quality increases and decreases along the depth offocus with the optimum image quality being somewhat centrally located.Accordingly, an envelope 933 is formed representing the image quality ofthe columns 930 of vertical lines. Similarly illustrated in FIG. 11B,the image quality of columns 932 of horizontal lines is represented bybars 932′, with the height of the bars 932′ in the z directionrepresenting image quality. The image quality increases and decreasesalong the depth of focus in the x direction. Accordingly, an envelope935 of the bars 932′, can be determined representing the image qualityof the columns 932 of horizontal lines on the reticle portion 916,illustrated in FIG. 11A. The image of the columns 930 of vertical linesrepresented by bars 930′ are interlaced between the image of the columns932 of horizontal lines represented by bars 932′. If there is noastigmatism at the field location of the optical system beingcharacterized, the envelopes 933 and 935 will coincide. However, anyastigmatism can be detected by a relative shift between the envelopes933 and 935, represented by distance a′.

FIGS. 9A and 9B and FIGS. 11A and 11B illustrate different techniques toobtain the same information using different embodiments of the presentinvention. The teachings of the present invention, in simultaneouslyimaging a plurality of different feature sets, periodic patterns, orgratings on a reticle and recording the resulting images in a volume ofspace, makes possible the detection and characterization of aberrationsof the optical system in a single step or exposure. The teachings of thepresent invention can be utilized to determine different aberrations inthe optical system depending upon the different feature sets, periodicpatterns, or gratings utilized on portions of the reticle.

FIG. 12 illustrates a portion of a reticle 1016 having a feature set orline pattern that can be utilized to detect spherical aberrations. Thereticle portion 1016 represents columns 1030 and 1032 of alternatinglines with different line spacing or width. For example, the linespacing of column 1030 can be 300 nanometers and the line spacing ofcolumn 1032 can be 100 nanometers. The reticle pattern portion 1016illustrated in FIG. 12 is analogous to the reticle pattern portion 716illustrated in FIG. 9A. However, where the reticle pattern portion 716utilizes line orientation to detect astigmatism, the reticle patternportion 1016 utilizes linewidth or spacing to detect sphericalaberrations. All detect the image of the respective reticle patternportion in a volume of space at different depths of focus, such as whena photosensitive substrate is tilted in the image volume of space.Additionally, all can be read with an interferometer in a single stepwith the different imaged lines containing information representative ofthe aberrations of the optical system. For the reticle pattern portion1016, the image quality will vary along the depth of focus for thedifferent linewidths. Accordingly, an envelope representing the imagequality as a function of the depth of focus for each different linewidthsection will shift depending upon any spherical aberrations. It shouldbe appreciated that different reticle portions can be utilized havingdifferent line patterns over portions of the reticle to detect a varietyof different aberrations at different locations in the field. Thesedifferent portions of reticle patterns can be incorporated in a singlereticle to simultaneously detect and measure the field curvature anddifferent aberrations.

FIG. 13 represents a reticle 1117 that is divided into a plurality ofdifferent sections, having as an example thereof section 1119 a, 1119 b,1119 c, and 1119 d among other sections that can have different reticlepattern portions configured to detect different aberrationssimultaneously over a field to characterize the optical system. Forexample, magnification can be measured as the angle ofretro-diffraction. Normal feature pitch and associated nominaldiffraction beam angle can be measured differently from a calibratednominal pitch substrate or calibrated prism or nominal angle betweenfaces. Distortion, residual after magnification removal, can be measuredas the scaled phase map residual. The scaling reflects the relationshipbetween in-plane distortion, IPD, and the geometric constraints of thenormal periodic pattern or grating pitch, the interferometer wavelength,and the local retro-diffraction beam angle. Coma can be measured by aninduced image shift through focus seen as a second-order distortionacross the field tilted through the depth of focus of the opticalsystem.

FIG. 14 is a perspective view of an interferometric analysis or map of aresist covered or photosensitive substrate exposed with the image of abasket weave or interlaced or cross periodic pattern or grating.Alternatively, such an analysis could be performed by viewing thepattern in real time by using a demodulating device (such asdemodulating device 24 as shown in FIG. 1B). The basket weave or crossperiodic pattern or grating is a reticle having orthogonal lines overthe entire field. The entire field of the optical system can becharacterized by exposing a reticle over the field onto a tiltedphotosensitive substrate. The photosensitive substrate should be tiltedso that the entire field falls within the depth of focus of the opticalsystem. Because of the tilt, the x axis in FIG. 14 represents focus andfield position in the x direction. The y axis represents field positionin the y direction. The z axis represents the change in pitch betweenthe lines in the periodic pattern or grating as a result of aberrationsor distortions of the optical system. The surface contour 1221 providesinformation of the imaging characteristics of the optical system. Theoptical system can be characterized globally by interpreting the entirefield, or locally by interpreting a desired portion of the field.

FIG. 15 is a diagram graphically depicting different imagingcharacteristics and distortions or aberrations that can be obtained tocharacterize the optical system using this embodiment of the presentinvention. Arrow 1202 represents coma and is illustrated by thegenerally or overall curved surface contour 1221 shown in FIG. 14. Arrow1204 represents telecentricity and is illustrated as a tilt in the x-yplane about the y axis of the surface contour 1221 shown in FIG. 14.Arrow 1206 represents overall or a mean magnification and is illustratedas a tilt in the x-y plane about the x axis of the surface contour 1221shown in FIG. 14. Arrow 1208 represents y-distortion signature or alocal change in magnification and is illustrated by the local changes inthe surface contour 1221 shown in FIG. 14. If there where no aberrationsor distortions over the entire field, the interferometric map wouldresult in a flat un-tilted surface.

FIGS. 16A-16D schematically illustrate in perspective view the differentdistortions or aberrations of the optical system being characterized andillustrated graphically in FIG. 15. FIG. 16A represents lines having atilt in the x-y plane about the x axis. This tilt represents global oroverall magnification. Accordingly, if there is no global or overallmagnification within the field, there is no tilt in the x-y plane aboutthe x axis. FIG. 16B represents lines having a curve or second order bowthrough focus. This curve through focus or the x direction representscoma. FIG. 16C represents lines having a tilt in the x-y plane about they axis. This tilt represents telecentricity. FIG. 16D represents lineshaving a local curve. This curve represents y distortion signature orlocal changes in magnification as a function of field position. All ofthese features or characteristics can be independently extracted fromthe interferometric map illustrated in FIG. 14. Accordingly, the entirefield of the optical system can be characterized in a single stepwithout the need for multiple exposures or separate analysis.

FIG. 17 is a plane view of an exposed photosensitive substrateillustrating an embodiment of the present invention for determining bestfocus of an optical system. The image of a reticle is projected onto aphotosensitive substrate 1322 over the field of an optical system.Alternatively, the image of the reticle could be presented for viewingin real time by using a demodulating device (such as demodulating device24 as shown in FIG. 1B). The reticle projects the image of a basketweave periodic pattern or grating pattern along the two longitudinaledges 1328 of a rectangular field. The photosensitive substrate 1322 istilted about the longitudinal axis, so that a relatively narrow firstband 1331 is printed laterally across the photosensitive substratewithin the two longitudinal edges 1328 during a first exposure. Thephotosensitive substrate 1322 is then shifted a known distance co-axialwith the optical axis, in the z direction, which extends into thedrawing sheet of FIG. 17, so that a relatively narrow second band 1331′is printed laterally across the photosensitive substrate within the twolongitudinal edges 1328 during a second exposure. The position of bestfocus for the optical system can be determined by analyzing thepositions of the first and second printed bands 1331 and 1331′. Theanalysis is performed using geometry that can be readily determined orderived based upon the known distance shifted. For example, the focusposition for the center of the field at point M is obtained by measuringthe distance OA and O′A′. These numbers yield the position of theexposed first printed bands 1331 relative to the known field center M.Interpolation of the focus values for the two exposures forming firstand second bands 1331 and 1331 ÿ yields the focus value for the fieldcenter at M. This focus value is along the optical axis only. Tilt errorabout the lateral axis is calibrated by measuring the distance AB alongthe substrate. The tilt slope is expressed in nanometers of focus shift,as determined by the focus difference between the two exposures, permillimeter of substrate, as determined by the distance AB. Using thistilt slope value the bow or tilt about the longitudinal axis error isdetermined by measuring the angle θ of the line A-A′ or B-B′ viameasurement of the distance difference between distance OA and O′A′ ordistance OB and O′B′. From the measurement of four of these distances,the substrate is aligned to best focus plane with redundancy formeasurement error correction or averaging. Alternatively, the values canbe extracted from the following formulas, where:

-   -   M′ lies on the midpoint of a line 1333 midway between line A-A′        and line B-B′;    -   IFS is the induced focus shift or intentional shift along the z        or optical axis between the two exposures;    -   IT is the induced tilt or intentional shift about the lateral        axis.

Then,

-   -   the slope (S) is equal to H/W;    -   the focus error (FE) is equal to IFS/AB×MM′;    -   the tilt error (TE) about the longitudinal axis is equal to        (IFS/AB)−IT; and    -   the tilt or bow error (BE) about the lateral axis is equal to        S×IFS/AB.

FIG. 18 illustrates the use of an embodiment of the present invention todetect spherical aberrations. Curve or line 1402 represents the resistdepth as a function of focus. Due to a tilt through focus when exposinga photosensitive substrate, the periodic pattern or grating formed onthe photosensitive substrate by the processed resist has a varyingdepth. The depth is greatest at best focus and becomes smaller as focusdegrades. The asymmetry in curve or line 1402, identified at region1404, is representative of spherical aberrations. Accordingly, thepresent invention can be applied to detect spherical aberrations in anoptical system.

FIGS. 19A and 19B illustrate another embodiment of the present inventionfor determining initial placement of a reticle in the optical system forobtaining optimized imaging. Referring to FIGS. 19A and 19B, aphotosensitive substrate 1522 is exposed by a reticle 1516.Alternatively, the image of the reticle is presented for viewing in realtime by using a demodulating device (such as demodulating device 24 asshown in FIG. 1B). The reticle is tilted out of an x-y object planeabout the x axis. The photosensitive substrate 1522 is preferably out ofthe x-y plane about the y axis. Accordingly, the reticle 1516 and thephotosensitive substrate 1522 are tilted orthogonal with respect to eachother, similar to the embodiment illustrate in FIG. 1. The reticle 1516has a plurality of orthogonal interlaced lines with differentlinewidths. For example, line 1531 has a relatively narrow verticallinewidth and line 1533 has a relatively wide vertical linewidth. Thevertical lines 1531 and 1533 are alternating or interlaced in the xdirection. Relatively narrow horizontal line 1534 and relatively widehorizontal line 1536 are alternating or interlaced in the y direction. Agrid pattern of alternating or interlaced horizontal and vertical linesof different widths is thereby formed. The grid pattern on the reticle1516 is imaged through reticle position, due to the tilt in the reticle1516, onto the photosensitive substrate 1522 through focus, due to thetilt in the photosensitive substrate 1522, during an exposure. Theprocessed photosensitive substrate 1522 will have a locus of best focusposition as a function of linewidth or feature size. This locus isdetermined by examining the image, including the resist depth.Generally, the maximum resist depth determines best focus.Alternatively, the locus of best focus position could be determined byanalyzing the visible pattern produced by the demodulating device (suchas demodulating device 24 as shown in FIG. 1B). That is at best focus,the resist is more fully exposed and therefore has greater depth. Theposition at which the locus of best focus position for each differentlinewidth cross represents the preferred position for the reticle tominimize aberrations, and in particular spherical aberrations. Referringto FIG. 19A, the intersection of lines 1502 and 1504 represents theoptimum position for the reticle 1506 to minimize spherical aberrations.Line 1506 represents the location or plane of optimum position for thepositioning of the reticle 1516 to obtain the best image or minimumspherical aberrations. For example, as illustrated along the leftlongitudinal edge of the photosensitive substrate in FIG. 19A, if thereticle 1516, in FIG. 19B, was tilted about the x axis one unit, theline 1506 indicates that the reticle should be positioned at 0.4 unitsto obtain the best or optimum imaging. The line 1506 is drawn parallelto the axis of tilt of the reticle, or x axis. While only two differentalternating or interlaced linewidths have been illustrated, it should beappreciated that any number of different linewidths can be alternatingor interlaced.

While the present invention has been illustrated and described withrespect to different embodiments and different feature sets or linepatterns, clearly other feature sets or line patterns can be utilizedand arranged in different ways to characterize an optical system.However, all of the embodiments of the present invention simultaneouslyimage a variety of different pattern portions in a volume of space atdifferent depths of focus. The recorded images of the plurality ofpattern portions at different depths can be interferometrically analyzedso as to characterize the optical systems. This interferometric analysisis preferably accomplished in a single step such that the data obtainedfrom the interferometric analysis of the recorded image of the reticleprovides nearly complete characterization of the optical system. Thepresent invention therefore prevents the need to sequentially select andanalyze different locations within the field of the optical system. As aresult, the teachings of the present invention result in a very rapidand robust characterization of the optical systems.

Accordingly, it should be appreciated that the method and apparatus ofthe present invention makes possible the characterization of an opticalsystem in a single exposure or imaging step or real time viewing todetermine focus, field curvature, astigmatism, coma, and/or focal planedeviations of the optical system. The present invention is particularlyapplicable to the characterization of photolithographic lenses used inprinting mask or reticle patterns onto a photosensitive substrate. Thepresent invention determines the best focus by detecting the envelope offeature quality through focus, rather than through the evaluation ofimage quality or line quality in a three-dimensional array of individualsample points in x, y, and focus. The present invention yields acontinuum of data through focus and reticle object position.

Therefore, the present invention has the advantage of being focusself-seeking; that is, it is highly insensitive to normal focal planelocation errors in that it will always print the zone of best focus ifthe wafer or photosensitive substrate field being exposed intercepts thedepth of focus. The present invention has the advantage of being highlysensitive and having low noise and a single exposure providing rapidacquisition of characterizing parameters. The present inventioneliminates the need for focal plane slicing with its associated timeconsuming multiple exposures and focus slicing errors.

In testing, sensitivity and noise levels have been obtained routinely atless than the five nanometer level. These low levels cannot be obtainedusing prior techniques. Prior techniques generally degrade withdecreasing linewidth. However, the present invention has the advantagethat it becomes more robust as linewidth decreases. This occurs becausethe present invention relies on resolving the envelope of featurequality rather than linewidth image.

The present invention can also obtain full field data in seconds, arelatively short time. This is an important feature in lithographictools using deep UV and beyond because of the small line sizes andthermally varying time constants. The ability of the present inventionto utilize a full field exposure in a single shot eliminates alignmenttiming errors due to the scanning acquisition of data. The use of theplurality of different feature sets having multiplexed featureorientations, sizes, and line types allows for the determination offocus position, astigmatism, field curvature and depth of focus.Additionally, the present invention can yield information on coma,spherical, and variation of coherence.

The present invention, in consisting of multiplexed periodic featuresthat are imaged by the imaging system to be tested and a lithographicrecording process, including a metrology tool to analyze the printedimages makes possible the rapid characterization of an optical system.The feature sets can be a group or isolated variant line types, shapes,sizes and orientations. The present invention images these feature setsthrough and beyond the depth of focus of the imaging system in a singleexposure. The envelope or feature quality through focus is printed andanalyzed. This analysis can consist of full depth of focus dataevaluation, as in the case of auto-correlation and cross-correlationanalysis. Alternatively, the analysis can identify envelope maxima orminima asymmetry or slope. This is contrary to the prior techniques thatanalyze individual features at pre-determined and consequentlynon-optimum discrete focal positions.

The quality of particular feature sets through focus can be used todetermine flat focus, field curvature, astigmatism, sphericalaberration, partial coherence, distortion and coma, depending upon thefeature type orientation and/or size selected. In a case of astigmatism,different line orientations can be interlaced down the field and read bya dark field or interferometric microscope. Alternatively, differentline orientations can be interlaced across the field and read by aninterferometric microscope or atomic force microscope. In the case ofdistortion, the features can be read using a full field interferometer.

Accordingly, it should be appreciated that the present invention greatlyadvances the ability to characterize quickly and easily an opticalsystem and in particular projection optics used in photolithography forthe manufacture of semiconductor wafers. From a single exposure, dataacquisition, or viewing step, valuable information can be obtainedcharacterizing the optical system at a single point in time. Thisgreatly increases throughput and yield in that imaging performance ismaintained at a high level.

2. Holographic Test Reticles

As described herein, a test reticle with a plurality of periodicpatterns and other structures is used to characterize an optical system(such as a lens) under test. For example, as shown in FIG. 1, theoptical system 18 is imaged with the pattern on the test reticle 16,resulting in image data that is recorded on the substrate 22. Thesubstrate 22 is examined to retrieve image data that is subsequentlyprocessed to determine parameters for the optical system including:focus, field curvature, astigmatism, coma, focal plane deviation,spherical aberration, and coherence variation.

Since the test reticle 16 is used to test the quality of the opticalsystem 18, it is preferable that the patterns on test reticle 16 be asaccurate as possible so that a true characterization can be made. Morespecifically, it is important that the lines and spaces of the gratings(e.g., see FIG. 4) on the test reticle have accurate dimensions andplacement. If the grating are not accurate, then it is difficult todetermine if aberrations recorded on the substrate 22 are caused by theoptical system 18 or by the test reticle 16.

Conventional means for making reticles, including test reticles, includee-beam writing tools and laser writing tools. These conventionaltechniques typically write sub-fields of a larger pattern that aresubsequently stitched together to create the larger composite fieldpattern. When the sub-fields are stitched together, reticle writingerrors can occur. At sub-100 nm linewidths in very high numericalaperture (VHNA) lithographic tools, these writing errors have become alimiting factor in the ability to test optical imaging systems.

Hence, the following discussion describes a method and system forfabricating holographic test reticles in accordance with the presentinvention. Holographic reticles are generated by interfering two or morebeams of optical radiation to generate an interference volume having aperiodic interference pattern, such as the gratings and other teststructures described above. The interference patterns are then recordedon a reticle blank using various recording techniques such asphoto-resist, etc. The geometry of the interference patterns is tightlycontrolled by the properties of the interfering optical beams. Morespecifically, the geometry is controlled by the wavelength of light, thewavefront variation, and the geometry of the exposure configuration(i.e., the relative beam angle of the optical radiation before and afterinterference). All of these factors can be controlled much moreprecisely than serially written e-beam or laser writing techniques.Additionally, much larger reticle areas can be written in a single passusing this holographic technique. As such, writing errors that resultfrom stitching together e-beam sub-fields are avoided entirely.

FIG. 20 illustrates a system 2000 for writing a holographic reticle, andincludes: a laser 2002, a splitter 2006, wavefront manipulation optics2010, an interference volume 2012, and a reticle blank 2016 havingphoto-resist 2014. The system 2000 is described with reference to aflowchart 2100 (FIG. 21), as follows.

In a step 2102, the laser 2002 generates coherent optical radiation2004.

In a step 2104, the splitter 2006 splits the optical radiation 2004 intotwo or more beams 2008 a,b. Two beams 2008 a,b are shown for ease ofillustration. However, multiple beams 2008 could be generated, where thenumber of beams is dependent on the type of interference pattern that isdesired.

In a step 2106, the wavefront manipulation optics 2010 manipulate thewavefronts of one or more of the beams 2008, resulting in beams 2011a,b. Exemplary optics 2010 include various optical components that aregenerally used to alter the wavefronts of laser beams, and include butare not limited to: lens, expanders, collimators, spatial filters,mirrors, etc. As a specific example, spatial filtering of sphericalwaves that are produced by beams converging through a pinhole willgenerate tightly controlled wavefronts that are dictated by thewavelength, wavefront divergence angles, propagation distances, and beamintersection angles. Additionally, the optics 2010 are aligned so thatthe resulting beams 2010 will subsequently interfere and produce aninterference volume.

The resulting beams 2011 a,b have wavefronts that generate a desiredinterference pattern during subsequent beam interference. The specifictype of wavefront for each beam 2011 depends on the specificinterference pattern that is desired. Exemplary wavefronts include butare not limited to: cylindrical wavefronts, planer wavefronts, sphericalwavefronts, etc. Specific wavefront combinations and associatedinterference pattens are discussed further herein.

In a step 2108, the beams 2011 a,b interfere to produce an interferencevolume 2012 having an associated interference pattern. FIG. 20illustrates two beam interference for ease of illustration. However, thescope of the invention includes multiple beam interference, where thenumber of beams depends on the type of interference pattern that isdesired.

In a step 2110, photo-resist 2014 on the reticle 2016 records theinterference pattern that is associated with the interference volume2012. Other types of recording mediums could be used including but notlimited to:

-   -   photographic film, holographic film, photo-refractive media,        photopolymers, and other known means for recording an        interference pattern that will be understood by those skilled in        the relevant arts.

In a step 2112, the photo-resist 2014 is developed to generate a testreticle having the desired interference pattern.

In a step 2114, an optical system is tested using the holographic testreticle, such as the optical system 18 that was described in FIG. 1.

There are many advantages to writing test reticles holographically, someof which are discussed as follows. First, holographic patterning is moreaccurate than e-beam techniques because the resulting interferencepattern is determined by the wavelength of light, the wavefrontvariation of the interfering beams, and the geometry of the exposureconfiguration. All of these factors can be controlled more accuratelythan in conventional e-beam and laser writing techniques, and therebyreducing reticle writing errors that are associated with conventionaltechniques.

Due to the increased accuracy, the periodic structures (e.g., grating)that are common to optical performance testing are easily produced usingholographic techniques. For example, in linear gratings, the linewidthpitch uniformity can be precisely controlled, and therefore distortionis minimized. Additionally, in chirped gratings, variable pitch patternscan be produced with great accuracy. Therefore, an optical system can betested over a precisely-controlled continuum of linesizes, lineorientations, and pattern pitches. Additionally, phase shifts inperiodic structures can be precisely controlled. Phase shifted gratingsare useful for the characterization of odd optical aberrations inoptical systems, which produce feature shifts in the image plane.

Additionally, holographic patterning can print linewidths that are muchsmaller than the current reticle writing tools, including e-beam andlaser writing tools. For example, e-beam techniques are currentlylimited to 100 nm and above, whereas holographic patterning can printlinewidths that are sub-100 nm and as low as 50 nm.

2a. Specific Configurations and Interference Patterns

Specific embodiment for patterning holographic reticles and theresulting interference patterns are described as follows. Theseembodiments are meant for example purposes only and are not meant to belimiting. Other example embodiments will be understood by those skilledin the arts based on the discussion given herein. These other exampleembodiments are within the scope and spirit of the present invention.

FIG. 22A illustrates an example of holographic reticle patterning (orwriting) based on interference of two spherical beams. Referring to FIG.22A, optical expanders 2204 a,b receive optical radiation beams 2202 aand 2202 b. The expanders 2204 a,b manipulate the beams 2202 a,b to haveexpanding spherical wavefronts, represented as beams 2206 a and 2206 b.The beams 2206 a and 2206 b interfere to produce an interference volume2208 having a substantially linear grating pattern as shown. The lineargrating pattern is recorded on a reticle blank 2210. The linewidth andspacing of the grating pattern (also called pitch uniformity) aretightly controlled by the wavelength of the beams, and the angle atwhich the beam interference occurs. When lasers are used as the opticalsource, the optical wavelength is extremely accurate and stable.Therefore, the pitch uniformity of the resulting grating is also veryaccurate and stable, and improved over that achieved with e-beam orlaser writing techniques.

In FIG. 22A, spherical expanding beams are illustrated to create lineargratings for example purposes only, and are not meant to be limiting.Other embodiments will be understood by those skilled in the arts. Forexample, long path length quasi-plane wave beams can be used to improvethe pitch uniformity. Alternatively, additional optics can be utilizedto collimate the beams to produce plane waves. In other words, lineargratings can be produced by interfering collimated light.

FIG. 22B illustrates a simulation associated with an interferencepattern 2212 that is produced by spherical two beam interference. Thesimulation represents the change in pitch uniformity over the pattern2212. The box 2214 in the center of the interference pattern 2212highlights an area having constant pitch uniformity. In other words, thelinewidths and spaces are substantially constant within the box 2214. Incontrast, the box 2216 highlights an area of the pattern 2212 having avariable (but controlled) pitch uniformity. More specifically, thelinewidths and spaces in the box 2216 are increasing, but at a known andcontrolled rate. This is known as a chirped grating. Similarly, otherparts of the pattern 2212 have linewidths and spacing that aredecreasing at a controlled rate.

FIGS. 23A-E illustrate holographic reticle patterning for generating aninterference pattern having a chirped grating. As mentioned above,chirped gratings have a series of continuously variable lines andspaces, as further illustrated in FIGS. 23B-E. Chirped gratings areuseful for determining image distortion of an optical systems overmultiple linewidths and spacings, without requiring multiple exposures.

Referring to FIG. 23A, holographic reticle configuration 2300 depicts anoff-axis cylindrical and plane wave beam combination, which is usefulfor generating interference patterns that have a chirped grating.Radiation beams 2310 a,b, having planer wavefronts, are projected ontothe bottom of a reticle 2306, as shown. Additionally, a mirror 2302projects radiation beams 2304 a,b onto the reticle 2306 at angles ÿ andÿ, relative to the beams 2310 a,b. The beams 2304 a,b preferably have acylindrical wavefront, and meet at a point 2308. The beams 2310 a,b and2304 a,b interfere to produce an interference volume having a chirpedgrating pattern, where the characteristics of the chirped grating aredictated by the geometry of the cylindrical divergence and theinterfering beam wavelengths.

FIGS. 23B-E illustrate exemplary chirped gratings. More specifically,FIG. 23B illustrates a cylindrical zone plate grating 2312, where thelinewidths and spacings are a maximum at the center of the grating, anddecrease from the center of the grating to the edge of the grating. FIG.23C illustrates a reverse cylindrical zone plate grating 2314, where thelinewidths and spacings are a minimum at the center of the grating andincrease to maximum at edges of the grating. FIG. 23D illustratesinterlaced chirped grating 2316 composed of multiple chirped gratings2318 a-e. The interlaced grating 2316 is generated by taking multipleexposures of the component gratings 2318, and moving the reticle blankin the y-direction between exposures. The interlaced grating 2316enables image distortion to be measured at multiple field points of theoptical system under test, simultaneously. FIG. 23E illustrates acircular zone plate array.

As mentioned above, the characteristics of holographically generatedchirped gratings (such as pitch variation) are dictated by the geometryand wavelength of the interfering beams. As a result, theholographically generated chirped gratings are continuous and smoothlyvarying across their extent. In contrast, discrete patterning methodstypically vary the linewidth and pitch of a grating as a function ofscanned, rastered, or pixelated patterning. These discrete serialmethods suffer from temporal variations in patterning beam location,stage location, and stitching accuracies.

FIG. 23F illustrates focus determination of an optical system using theinterlaced chirped grating 2316. More specifically, focus curves 2320a-e are generated by imaging an optical system under test (such asoptical system 18 in FIG. 1) using the interlaced chirped grating 2316.Each curve 2320 represents the depth of focus (in the z-direction ofFIG. 1A) that corresponds to the linewidths in the adjacent grating2318. One of the linewidths in the gratings 2318 is arbitrarily selectedto provide a reference focus (such as linewidth 2322), and the depth offocus for the other linewidths are plotted relative to the referencefocus, as shown.

FIG. 24 illustrates an atomic force micrograph of an actualcross-grating 2400 that was patterned on a holographic test reticle. Thecross grating 2400 is viewed at an angle of 45 degrees, and has two setsof orthogonal lines (i.e., 2-fold geometry).

FIG. 25 illustrates a holographic hex pattern 2500 having lines in threedifferent orientations (i.e., 3-fold symmetry), and therefore allowingimage distortion at these orientations to be measured simultaneously.This allows the image distortion of an optical system to be measured atthese three orientations, simultaneously. The invention is not limitedto 3-fold symmetry, as n-fold symmetry will be discussed below.

FIG. 26 illustrates a polygonal grating 2600 that is the resultinterfering/combining multiple plane wave beams. The grating 2600 iscomposed of intersecting lines in the x-y plane that intersect at therelative angles of 0, 45, 90, and 135 degrees. The invention is notlimited to this geometry. As will be shown below, multi-beaminterference can be used to generate complex sub-micron geometries on areticle that have 2-fold, 3-fold, 4-fold, and in general n-foldgeometries. These n-fold patterns can be used to probe optical systemparameters that are dependant on line orientation. The advantage ofproducing these patterns holographically is that the spatialrelationships between the periodic structures are tightly constrained.Additionally, these n-fold patterns are valuable in decoupling thecollection of distortion, coma, or other image shifting aberrations thatare separable as azimuthally-dependent asymmetric aberrations in thepupil plane of the optical system.

In embodiments, the intersecting lines in the grating 2400 have asinusoidal amplitude whose intensity varies according to the function: [sin (x+y)*sin(x−y)*sin(x)*sin(y)]². Other intensity distributions couldbe used, including binary on-off lines, as will be understood by thoseskilled in the arts. These other amplitude functions are within thescope and spirit of the present invention.

FIG. 27 illustrates a zone plate array 2700, which takes the n-foldgeometry to the limit. Zone plate array 2700 includes circles havingvariable linewidths and spacings (i.e., chirped). Because of thecircular orientation of array 2700, image distortion for all possibleline orientations can be measured simultaneously, and with a single testreticle. In embodiments, the zone plate array 2700 is generated bycombining/interfering two beams of optical radiation so as to create aspherical beam.

2b. Phase Shifting Interference Patterns

FIG. 28 illustrates holographic patterned gratings 2802-2806, whichcollectively depict examples of pitch change and phase change forperiodic gratings. More specifically, gratings 2802 and 2804 depictpitch change because the linewidths and spacings (i.e., pitch) of thegrating 2802 are much smaller than the pitch of the grating 2804.Gratings 2804 and 2806 depict phase change because the lines of thegrating 2806 are shifted in the x-direction relative to the lines in thegrating 2804.

Alternatively, the duty cycle (or line-to-space ratio) of the gratingcan be varied continuously or in steps while maintaining the samegrating pitch. FIG. 29A illustrates a reticle with a holographic patternhaving a constant pitch grating with a varying duty cycle. In FIG. 29A,a reticle 2900 has a constant pitch grating 2902. However, at a ProfileA 2904, the duty cycle of grating 2902 is a 1:1 line-to-space ratio,while at a Profile B 2906, the duty cycle of grating 2902 is a 3:1line-to-space ratio. Having such a holographic pattern with a constantpitch on one reticle 2900 allows the entire pattern to beinterferometrically interrogated at a single diffraction angle. Bytesting a photolithographic system with a set of holographic patternseach with a constant pitch on one reticle, aberrations can be identifiedand decoupled based on the various induced image shifts and focus shiftsproduced as a function of the line-to-space duty cycle.

FIGS. 29B and 29C illustrate how the holographic pattern of reticle 2900is formed. FIG. 29B illustrates a uniform grating pattern 2908. FIG. 29Cillustrates a pattern with variation in exposure intensity 2910. In FIG.29C, exposure intensity is greater at a bottom portion 2912 and lesserat a top portion 2914. Exposure intensity between bottom portion 2912and top portion 2914 varies continuously in a gradual transition betweenthe values of these two portions 2912, 2914. Holographic reticle pattern2900 is formed by superimposing uniform grating pattern 2908 on thepattern with variation in exposure intensity 2910. One skilled in theart would recognize that any of a variety of duty cycle patterns couldbe produced by superimposing a pattern with a variation in exposureintensity on a uniform grating pattern. Particularly, the pattern withthe variation in exposure intensity is not limited to one in which theexposure intensity transitions from a high value to a low value acrossthe span of the pattern.

FIG. 30 illustrates a phase control system 3000 for creating acontrolled phase shift, such as the phase shift between the grating 2804and 2806 in FIG. 28. The control system 3000 includes optical detectors3004 and 3006, a control input 3008, and a difference module 3010. Thecontrol system 3000 can be operated in a fringe locking capacity, and/orcan be used to implement an intentional grating phase shift, based onthe control signal 3008.

The optical detectors 3004 and 3006 are placed to measure lightintensity at different points of a holographic interference pattern3002, resulting in intensity signals 3005 and 3007. In embodiments ofthe invention, the detectors 3004 and 3006 are optical detector diodesor equivalent devices that generate an electrical signal that isproportional to the intensity of the detected light.

The difference module 3010 receives the intensity signals 3005 and 3007,and the control signal 3008. The difference module 3010 determines adifference signal 3011 by adding the control signal 3008 to theintensity signal 3005, and then subtracting the intensity signal 3007.During fringe locking, the control signal 3008 is substantially zero,and therefore the difference signal 3011 represents the differencebetween the light intensity that is measured by the detectors 3004 and3006. If the difference signal 3011 is approximately zero, then thedetectors 3004 and 3006 are receiving approximately the same lightintensity, and therefore are monitoring the same corresponding locationon each fringe. If the difference signal 3011 is not approximately zero,then the detectors 3004 and 3006 are not straddling an equivalentportion of a light fringe.

The difference signal 3011 is used to control a mirror, or a crystal, oranother optic device (not shown) that phase shifts one of theinterfering beams that was used to create the interference pattern 3002.If the difference signal 3011 is approximately zero, then no action istaken. If the difference signal 3011 is not zero, then an interferingbeam is phase shifted in order to phase shift the interference (orfringe) pattern 3002. For fringe locking, the fringe pattern 3002 isphase shifted so that the detectors 3004 and 3006 will detect equivalentintensities of light, and therefore drive the difference signal 3011 toapproximately zero. Fringe locking is useful for making smallcorrections due to vibrations, and other random disturbances, etc. Incontrast to fringe locking, an intentional phase shift can be introducedin the fringe pattern 3002, even when signals 3005 and 3007 are equal,by introducing a non-zero control signal 3008, as discussed furtherbelow.

As illustrated in FIG. 31, the phase control system 3000 can beincorporated into a reticle writing system 3100 to write a holographictest reticle having gratings with a controlled relative phase shift. Thereticle writing system 3100 is similar to the system 2000 (FIG. 20),with the addition of the phase controller 3000 and the phase shiftingdevice 3102. The phase controller 3000 analyzes the interference volume2012, and generates the difference signal 3011 based on the interferencevolume 2012 and the control signal input 3008, as discussed above. Thedifference signal 3011 controls the phase shifting device 3102 in themanipulation optics 2010, which phase shifts the beam 2011 a and therebyproduces a phase shift in the interference volume 2012 that is based onthe control signal 3008. Therefore, the interference volume 2012 can bephase shifted by various amounts by changing the control signal 3008.The phase shifting device 3102 can be a mirror, a crystal, or anotheroptic device that is useful for phase shifting an optical beam. Otherspecific embodiments for the device 3102 include the following: areflective, refractive, or diffractive array; an electro-deformabledevice; an acousto-optic device; a nano-actuated optic device such as,but not limited to, a piezo-driven mirror or a bimorph-driven mirror; anano-deformable mirror array that is reflective, diffractive, orrefractive; a MEMS mirror array; an electro-deformable hologram; and anelectronic fringe-locking system.

Additionally, multiple phase shifted gratings (as shown in FIG. 28) canbe generated using the system 3100, by exposing the reticle blankmultiple times using different voltages for the control signal 3008.Flowchart 3200 (FIG. 32) describes the generation of multiple phaseshifted gratings in further detail.

Referring to FIG. 32, in step 3202, the voltage of the control signal3008 is set to a reference voltage. In step 3204, a reticle blank isexposed with a holographic interference volume to record a referencegrating that corresponds to the reference voltage. In step 3206, thereticle blank is moved in a direction that is perpendicular to thedesired phase shift of the grating. For example, in FIG. 28, if grating2804 was printed first, then the reticle would be moved in the ydirection, to print the grating 2806. In step 3208, the voltage of thecontrol signal 3008 is changed to effect a phase shift in theholographic interference volume. In step 3210, the reticle blank isre-exposed with the (phase shifted) holographic interference volume torecord a grating that is phase shifted relative to the reference gratingthat was generated in step 3204. The steps 3206-3210 can be repeatedmultiple times to generate multiple gratings having a relative phaseshift. Using this technique, extremely accurate phase shifts betweengratings can be realized. In embodiments, a phase-shift of minutefractions of a linewidth can be achieved. For sub-micron linewidths, itis possible to achieve a controlled phase shift in the angstrom range.

2c. Reticle Reading Analysis

As discussed, the test reticles that are described herein are preferablyutilized to test optical systems. For example, as shown in FIG. 1, theoptical system 18 is imaged with a test reticle 16, where the testreticle 16 can be a holographically generated test reticle. Theresulting image data is recorded on a photosensitive substrate 22, whichcan be subsequently analyzed to extract information that characterizesthe optical system 18. Alternatively, the resulting image data can bepresented for viewing in real time by using a demodulating device.

As illustrated in FIG. 7, the photosensitive substrate 22 is analyzedusing interferometric techniques to determine properties of the opticalsystem under test. The resulting interferogram represents changes in aphase front of light that is interferometrically diffracted off theexposed substrate 22.

FIG. 33 illustrates an example interferogram 3300 that represents aphase front of the diffracted light for a 3×3 field array of an opticaldevice under test. The interferogram 3300 is composed of nine blocks(corresponding to a 3×3 array) that are delineated by horns, such asexemplary horn 3302. Each block is characterized by a tilt and piston,which quantify the aberrations and distortion in the array field of theoptical system under test. Non-uniform distortion parameters can beanalyzed based on local pistons and tilts. More specifically, the tiltrefers to the angle of the block and represents the magnification of thereflected light and the telecentricty of the optical system under test.The piston refers to the height of the block and represents translationdifferences of the reflected phase front and therefore phase shiftcaused by the optical system under test.

Once characterized, the distortions and aberrations for an opticalsystem can be plotted vs. optical linewidth. For example, graph 3400(FIG. 34) illustrates coma induced distortion vs. optical linewidth.Other optical system characteristics can be quantified and plotted vs.optical linewidth. These include, but are not limited to: Zemikeaberrations, focus, field curvature, astigmatism, coma, distortion,telecentricity, focal plane deviation, spherical aberrations, andcoherence variation. Thus, non-uniform distortion parameters can bedetected as a function of variation in linewidth. One skilled in the artwill recognize that a non-linear phase front can be realized on a singleholographic reticle by using a chirped grating structure.

Graph 3400 shows as an example of how image shifts can occur as afunction of linewidth and aberration type. Graph 3400 and similar graphsprepared for other optical system characteristics are prepared from dataobtained at the best focus position of the photolithographic system.However, the magnitude of the image offsets are greatly influenced bythe partial coherence (PC) of the optical illumination used to image thelithographic features. FIGS. 35A and 35B illustrate how partialcoherence affects the image offsets. Graph 3500A (FIG. 35A) illustratesimage shift as a function of focus for a variety of linewidths where thepartial coherence of the optical illumination is 0.6. Graph 3500B (FIG.35B) illustrates image shift as a function of focus for a variety oflinewidths where the partial coherence of the optical illumination is0.3. One skilled in the art would recognize that comparing the relativeshift differences due to the different partial coherence conditions isanother method of deconvolving higher order aberrations from lower orderones.

The measurement of the relative image shifts as a function of linewidthcan be simplified by using a reticle with various linewidths such thateach linewidth is an integral multiple of a fundamental linewidth size.Table 3600 (FIG. 36) illustrates this pattern. Table 3600 illustratesthe relationship between linewidth and order of diffraction fordifferent diffraction angles. The diffraction angles are represented asletters. For example, consider a reticle that has linewidths withdimensions of 100 nm, 200 nm, 300 nm, 400 nm, and 600 mm. In this case,the second order diffraction of the 200 nm linewidth would be at thesame angle as the first order diffraction of the 100 nm linewidth.Likewise, the third order diffraction of the 600 nm linewidth would beat the same angle as the first order diffraction of the 200 nmlinewidth. Thus, a reticle with a set of linewidths can be measured forrelative image shifts at the same interferometric angle. Under testconditions, this allows for all data to be collected at a single sampleangle. This improves the speed at which tests can be conducted. It alsoimproves the robustness and sensitivity of the data collected.

3. CONCLUSION

Example embodiments of the methods and components of the presentinvention have been described herein. As noted elsewhere, these exampleembodiments have been described for illustrative purposes only, and arenot limiting. Other embodiments are possible and are covered by theinvention. Such other embodiments will be apparent to persons skilled inthe relevant art(s) based on the teachings contained herein. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments, but should be defined onlyin accordance with the following claims and their equivalents.

1. A method for making a holographic reticle, the method comprising thesteps of: (1) receiving two beams of coherent optical radiation; (2)interfering said two beams, resulting in an interference volume ofoptical radiation having an interference pattern; and (3) recording saidinterference pattern in a recording medium.
 2. The method of claim 1,further comprising the steps of: (4) receiving a single beam of coherentoptical radiation; and (5) splitting said single beam into said two ormore beams of optical radiation.
 3. The method of claim 1, wherein saidinterfering step results in the interference pattern being useful forcharacterizing an optical system.
 4. The method of claim 1, wherein saidinterfering step results in the interference pattern comprising agrating.
 5. The method of claim 4, wherein said interfering step furthercomprises: causing the grating to have linewidths and spacings that arebased on properties of said interfering beams.
 6. The method of claim 4,wherein said interfering step further comprises: causing the grating tobe a linear grating having a substantially constant pitch.
 7. The methodof claim 6, wherein said interfering step further comprises: causing thelinear grating to have a duty cycle that varies.
 8. The method of claim4, wherein said interfering step further comprises: causing the gratingto have a plurality of lines having multiple orientations.
 9. The methodof claim 4, wherein said interfering step further comprises: causing thegrating to be a chirped grating having a controlled pitch variation. 10.The method of claim 4, wherein said interfering step further comprises:causing the grating to be a cross grating.
 11. The method of claim 4,wherein said interfering step further comprises: causing the grating tobe a polygonal grating.
 12. The method of claim 4, wherein saidinterfering step further comprises: causing the grating to be a zoneplate array.
 13. The method of claim 4, wherein said interfering stepfurther comprises: causing the grating to be a multiplexed grating withmultiple axes of symmetry of controlled pitch and pitch uniformity. 14.The method of claim 1, further comprising the step of: (4) manipulatinga wavefront of one or more of said beams prior to step (2), inaccordance with a desired interference pattern.
 15. The method of claim14, wherein step (4) comprises the step of expanding two of said beams,resulting in two diverging spherical wavefronts that interfere andproduce an interference pattern with a linear grating.
 16. The method ofclaim 14, wherein step (4) comprises the step of spatially filteringsaid one or more beams.
 17. The method of claim 14, wherein step (4)comprises the steps of: (a) manipulating a first beam to have acylindrical wavefront; and (b) manipulating a second beam to have aplanewave wavefront.
 18. The method of claim 17, wherein step (2)comprises the step of interfering said first beam and said second beam,thereby producing an interference pattern having a chirped grating witha controlled pitch variation.
 19. The method of claim 14, wherein step(4) comprises the steps of: (a) manipulating a first beam to have aspherical wavefront; and (b) manipulating a second beam to have aplanewave wavefront.
 20. The method of claim 19, wherein step (2)comprises the step of interfering said first beam and said second beam,resulting in a zone plate array.
 21. The method of claim 1, wherein step(3) comprises the step of generating a test reticle having saidinterference pattern.
 22. The method of claim 21, wherein step (3)comprises the steps of: (a) exposing photo-resist that is deposited on areticle with said interference pattern; and (b) developing saidphoto-resist so that said reticle reflects said interference pattern.23. The method of claim 1, wherein said interference pattern comprises agrating, further comprising the step of: (a) generating precision phaseshifts between adjacent grating patches to monitor image shiftingaberrations.
 24. The method in claim 23, wherein said step (a) comprisesthe step of: (I) phase shifting a holographic reference beam relative toan object beam.
 25. The method of claim 24, wherein said step (I) isperformed using an electro-deformable device.
 26. The method of claim24, wherein said step (I) is performed using an acoustic-optic device.27. The method of claim 24, wherein said step (I) is performed using anano-actuated optic device.
 28. The method of claim 27, wherein saidnano-actuated optic device is one of a piezo-driven mirror and abimorph-driven mirror.
 29. The method of claim 24, wherein said step (I)is performed using one of a reflective array, a refractive array, adiffractive array, a nano-deformable reflective array, a nano deformablerefractive array, and a nano deformable diffractive array.
 30. Themethod of claim 24, wherein said step (I) is performed using one of aMEMS mirror array and an electro-deformable hologram.
 31. The method ofclaim 24, wherein said step (I) is performed using an electronicfringe-locking system.
 32. A method for making a holographic reticle,the method comprising the steps of: (1) receiving two beams of coherentoptical radiation; (2) manipulating a wavefront of one or more of saidbeams in accordance with a desired interference pattern; (3) interferingsaid two beams, resulting in an interference volume of optical radiationhaving an interference pattern; and (4) recording said interferencepattern in a recording medium.
 33. The method of claim 32, wherein step(2) comprises the step of expanding two of said beams, resulting in twodiverging spherical wavefronts that interfere and produce aninterference pattern with a linear grating.
 34. The method of claim 32,wherein step (2) comprises the step of spatially filtering said one ormore beams.
 35. The method of claim 32, wherein step (2) comprises thesteps of: (a) manipulating a first beam to have a cylindrical wavefront;and (b) manipulating a second beam to have a planewave wavefront. 36.The method of claim 35, wherein step (3) comprises the step ofinterfering said first beam and said second beam, thereby producing aninterference pattern having a chirped grating with a controlled pitchvariation.
 37. The method of claim 32, wherein step (2) comprises thesteps of: (a) manipulating a first beam to have a spherical wavefront;and (b) manipulating a second beam to have a planewave wavefront. 38.The method of claim 37, wherein step (3) comprises the step ofinterfering said first beam and said second beam, resulting in a zoneplate array.